Open Data supplied by Natural Environment Research Council (NERC)

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

Aquatracka fluorometer

The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.

The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.

This sensor was originally designed to assist the study of marine photosynthesis. With the use of logarithmic amplication, the sensor covers a range of 6 orders of magnitude, which avoids setting up the sensor range for the expected signal level for different ambient conditions.

The sensor consists of a hollow PTFE 2-pi collector supported by a clear acetal dome diverting light to a filter and photodiode from which a cosine response is obtained. The sensor can be used in moorings, profiling or deployed in towed vehicles and can measure both upwelling and downwelling light.

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

RRS Challenger 61 CTD Data Documentation

Instrumentation

The CTD unit was a Neil Brown Mk. 3 incorporating a pressure sensor, conductivity cell, platinum resistance thermometer and a Beckmann dissolved oxygen sensor. This was mounted vertically in the centre of a protective cage approximately 1.5m square.

Attached to bars of the frame were an Aquatracka logarithmic response fluorometer and a Seatech red light (661 nm) transmissometer with a 25 cm path length.

Above the frame was a General Oceanics rosette sampler fitted with 12, 10 litre water bottles. These comprised a mixture of Niskin, general purpose Go-Flo and ultra-clean teflon lined Go-Flo bottles as dictated by sampling requirements. The base of the bottles were 0.75m above and the tops 1.55m above the pressure head. One bottle was fitted with a holder for twin reversing thermometers mounted 1.38m above the CTD temperature sensor.

Above the rosette was a PML 2-pi PAR (photosynthetically active radiation) sensor pointing upwards to measure downwelling irradiance. A second 2-pi PAR sensor, pointing downwards, was fitted to the bottom of the cage to measure upwelling irradiance. It should be noted that these sensors were vertically separated by 2m with the upwelling sensor 0.2m below the pressure head and the downwelling sensor 1.75m above it.

No account has been taken of rig geometry in the compilation of the CTD data set. However, all water bottle sampling depths have been corrected for rig geometry and represent the true position of the midpoint of the water bottle in the water column.

Operational procedure and data logging

On each cast the CTD was lowered to a depth of approximately 5 metres and held until the oxygen reading stabilised. It was then raised to the surface and lowered continuously at 0.5 to 1 m/s to as close as possible to the sea floor. The upcast was done in stages between the bottle firing depths.

Data were logged by the Research Vessel Services ABC data logging system. The deck unit outputs were sampled at 32 Hz by a microprocessor interface (the Level A) which passed time stamped averaged cycles at 1 Hz to a Sun workstation (the Level C) via a buffering system (the Level B).

Data processing

The raw data comprised ADC counts. These were converted into engineering units (Volts for PAR meters, fluorometer and transmissometer: ml/l for oxygen: mmho/cm for conductivity: °C for temperature) by the application of laboratory determined calibrations and salinity was computed using the algorithm in Fofonoff and Millard (1983). The data were submitted to BODC in this form.

Within BODC the data were reformatted on an IBM main-frame. At this stage transmissometer air readings recorded during the cruise were used to correct the transmissometer voltage to the manufacturer's specified voltage by ratio. The voltages were then converted to percentage transmittance (multiplied by 20.0) and dissolved oxygen converted to µM (multiplied by 44.66).

Next the data were loaded onto a Silicon Graphics workstation. A sophisticated interactive screening program was used to delimit the downcast, mark the depth range of water bottle firings and flag any spikes on all of the data channels.

The data were returned to the IBM and the downcasts loaded into a database under the Oracle relational database management system. At this stage percentage transmittance was converted to attenuance to eliminate the influence of instrument path length using the equation:

Attenuance = -4.0 * loge (% trans/100)

Calibration sample data were merged into the database and files of sample value against CTD reading at the bottle depth were prepared for the Principal Investigators to determine the calibrations. Due allowance was made for rig geometry. Note that CTD downcast values were generally used although the bottles were fired on the upcast. The validity of an assumed static water column for the duration of the cast was checked on the graphics workstation and upcast values substituted if necessary.

Sigma-T values were calculated using the algorithm presented in Fofonoff and Millard (1983). Oxygen saturations were computed using the equation of Weiss (1970).

Calibrations

For each cast the mean pressure reading logged whilst the instrument was in air was determined. The average of these, determined as -1.8 db, was added to each pressure value.

Two digital reversing thermometers were fired at the bottom of each cast. The mean difference, determined for all casts on the cruise, between the averaged calibrated readings and the CTD temperature, -0.007 °C, was added to the CTD temperatures.

A sample was taken from the bottom bottle of each cast and salinity was determined using a Guildline Autosal. The mean difference, determined for all casts on the cruise, between the bottle values and the CTD salinity, 0.077 PSU, was added to the CTD salinities.

Extracted chlorophyll values were log transformed and regressed against fluorometer voltages to give the calibration equation:

Chlorophyll (mg/m3) = exp (2.642*V - 5.341) (n=371; r2=55.8%)

Dissolved oxygen was calibrated against Winkler titration data for water bottle samples as a primary standard and calibrated data from the underway Endeco system as a secondary standard.

North Sea Project

The North Sea Project (NSP) was the first Marine Sciences Community Research project of the Natural Environment Research Council (NERC). It evolved from a NERC review of shelf sea research, which identified the need for a concerted multidisciplinary study of circulation, transport and production.

The ultimate aim of the NERC North Sea Project was the development of a suite of prognostic water quality models to aid management of the North Sea. To progress towards water quality models, three intermediate objectives were pursued in parallel:

Production of a 3-D transport model for any conservative passive constituent, incorporating improved representations of the necessary physics - hydrodynamics and dispersion;

Defining a complete seasonal cycle as a database for all the observational studies needed to formulate, drive and test models.

Proudman Oceanographic Laboratory hosted the project, which involved over 200 scientists and support staff from NERC and other Government funded laboratories, as well as seven universities and polytechnics.

The project ran from 1987 to 1992, with marine field data collection between April 1988 and October 1989. One shakedown (CH28) and fifteen survey cruises (Table 1), each lasting 12 days and following the same track, were repeated monthly. The track selected covered the summer-stratified waters of the north and the homogeneous waters in the Southern Bight in about equal lengths together with their separating frontal band from Flamborough head to Dogger Bank, the Friesian Islands and the German Bight. Mooring stations were maintained at six sites for the duration of the project.

Table 1: Details of NSP Survey Cruises on RRS Challenger

Cruise No.

Date

CH28

29/04/88 - 15/05/88

CH33

04/08/88 - 16/08/88

CH35

03/09/88 - 15/09/88

CH37

02/10/88 - 14/10/88

CH39

01/11/88 - 13/11/88

CH41

01/12/88 - 13/12/88

CH43

30/12/88 - 12/01/89

CH45

28/01/89 - 10/02/89

CH47

27/02/89 - 12/03/89

CH49

29/03/89 - 10/04/89

CH51

27/04/89 - 09/05/89

CH53

26/05/89 - 07/06/89

CH55

24/06/89 - 07/07/89

CH57

24/07/89 - 06/08/89

CH59

23/08/89 - 04/09/89

CH61

21/09/89 - 03/10/89

Alternating with the survey cruises were process study cruises (Table 2), which investigated some particular aspect of the science of the North Sea. These included fronts (nearshore, circulation and mixing), sandwaves and sandbanks, plumes (Humber, Wash, Thames and Rhine), resuspension, air-sea exchange, primary productivity and blooms/chemistry.

Table 2: Details of NSP Process cruises on RRS Challenger

Cruise No.

Date

Process

CH34

18/08/88 - 01/09/88

Fronts - nearshore

CH36

16/09/88 - 30/09/88

Fronts - mixing

CH56

08/07/89 - 22/07/89

Fronts - circulation

CH58

07/08/89 - 21/08/89

Fronts - mixing

CH38

24/10/88 - 31/10/88

Sandwaves

CH40

15/11/88 - 29/11/88

Sandbanks

CH42

15/12/88 - 29/12/88

Plumes/Sandbanks

CH46

12/02/89 - 26/02/89

Plumes/Sandwaves

CH44

13/01/89 - 27/01/89

Resuspension

CH52

11/05/89 - 24/05/89

Resuspension

CH60

06/09/89 - 19/09/89

Resuspension

CH48

13/03/89 - 27/03/89

Air/sea exchanges

CH62

05/10/89 - 19/10/89

Air/sea exchanges

CH50

12/04/89 - 25/04/89

Blooms/chemistry

CH54

09/06/89 - 22/06/89

Production

In addition to the main data collection period, a series of cruises took place between October 1989 and October 1990 that followed up work done on previous cruises (Table 3). Process studies relating to blooms, plumes (Humber, Wash and Rhine), sandwaves and the flux of contaminants through the Dover Strait were carried out as well as two `survey' cruises.

Table 3: Details of NSP `Follow up' cruises on RRS Challenger

Cruise No.

Date

Process

CH62A

23/10/89 - 03/11/89

Blooms

CH64

03/04/90 - 03/05/90

Blooms

CH65

06/05/90 - 17/05/90

Humber plume

CH66A

20/05/90 - 31/05/90

Survey

CH66B

03/06/90 - 18/06/90

Contaminants through Dover Strait

CH69

26/07/90 - 07/08/90

Resuspension/Plumes

CH72A

20/09/90 - 02/10/90

Survey

CH72B

04/10/90 - 06/10/90

Sandwaves/STABLE

CH72C

06/10/90 - 19/10/90

Rhine plume

The data collected during the observational phase of the North Sea Project comprised one of the most detailed sets of observations ever undertaken in any shallow shelf sea at that time.

Fixed Station Information

North Sea Project CTD Site CW

Site CW was one of 123 North Sea Project CTD fixed stations.

Casts were performed by 17 cruises between 06/05/1988 and 30/09/1990, the measurements collected lie within a box bounded by co-ordinates 55.49384°N, -0.88826°E at the southwest corner and 55.50758°N, -0.86062°E at the northeast corner.

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.